thanks dschreck, some good points there, like the idea of forcing a parent - child relationship.
You cant force the developer to implement the interface, whereas if an abstract is used, if for example there is a method in the abstract that the devloper needs to inherit, then he will have to extend the abstract, therefore he will inherit the rules thats why i prefer abstracts to interfaces (not just because of that).

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mysql> SELECT * FROM `users` WHERE `users`.`clue` > 0;
Empty set (0.00 sec)